1. Field of the Invention
The present invention relates generally to a sublimation type color printing sheet for a thermal transfer recording apparatus. More specifically, the invention relates to a sublimation type color printing sheet which can print an image having high light-proofing capability.
2. Description of the Prior Art
The thermal transfer recording apparatus has a heat unit, such as a thermal head for charging a heat on a thermal printing sheet. By varying the thermal load provided by the heat unit, an image having gradation can be transferred on an image recording medium, such as a paper.
Conventionally, there has been known a color thermal printing sheet for this type of application, which is prepared by selecting three specific primary color dyes among sublimation type dyes.
In the prior art, as these three primary color dyes, those of a high density type azo type chromophore group which has a large molecular extinction coefficient and high sensitivity, have been principally selected. However, such azo type chromophore can sensitively cause cleave decomposition of unsaturated double coupling in --N.dbd.N-- by singlet oxygen or reduction gas due to irradiation of ultraviolet ray to degrade the original function of the azo type chromophore to cause variation or fading of color. Therefore, such conventional thermal color printing sheets are not at all satisfactory from the viewpoint of light-proofing capability.